PhysicsLAB AP Free Response Question
2008 Form B - B3

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A student is measuring the magnetic field generated by a long, straight wire carrying a constant current. A magnetic field probe is held at various distances d from the wire, as shown above, and the magnetic field is measured. The graph below shows the five data points the student measured and a best-fit curve for the data.
 
 
Unfortunately, the student forgot about Earth’s magnetic field, which has a value of 5.0 x 10-5 T at this location and is directed north.
 
 (a) On the graph, plot new points for the field due only to the wire.

 (b) Calculate the value of the current in the wire.

Another student, who does not have a magnetic field probe, uses a compass and the known value of Earth’s magnetic field to determine the magnetic field generated by the wire. With the current turned off, the student places the compass 0.040 m from the wire, and the compass points directly toward the wire as shown below. The student then turns on a 35-A current directed into the page.
 
 
 (c) On the compass, sketch the general direction the needle points after the current is established.

 (d) Calculate how many degrees the compass needle rotates from its initial position pointing directly north.

The wire is part of a circuit containing a power source with an emf of 120 V and negligible internal resistance.
 
 (e) Calculate the total resistance of the circuit.

 (f) Calculate the rate at which energy is dissipated in the circuit.




 
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